This invention relates to heart valves and more specifically to a reinforced material from which heart valve leaflets can be made with reinforcing fiber strands aligned along lines of stress, thus to dramatically increase the durability and longevity of the valve.
Artificial heart valves have been known for years and have been used to replace native valves that have become faulty through disease. The artificial heart valves themselves should ideally be designed to last for the life of the patient, in many cases in excess of thirty-five years, equivalent to over 1.8 billion heartbeats. Heart valves that can be replaced include aortic and pulmonary valves, as well as mitral and tricuspid valves.
As to the operation of normal heart valves, they open and close largely passively in response to changes in pressure in the heart chambers or great vessels i.e. aorta and pulmonary artery, which they connect. For example, the aortic valve situated between the left ventricle and the ascending aorta, opens when the rising pressure in the contracting left ventricle exceeds that in the aorta. Blood in the ventricle is then discharged into the aorta. The valve closes when the pressure in the aorta exceeds that in the ventricle.
Problems occur with the native valves when they fail to function properly through disease or trauma. Faulty valves exhibit leakage in the closed position, i.e. regurgitation, obstruction to flow in the open position, i.e. stenosis, or a combination of the two, i.e. mixed valve disease. The response of the heart to faulty valves is demonstrated by changes in the left ventricle which ensue in response to malfunction of the aortic valve. Initially the heart compensates by an increase in muscle mass i.e. hypertrophy, a process that is to some extent reversible. Eventually, however, the heart can compensate no longer and begins to dilate. This latter process is irreversible even with replacement of the faulty valve. Untreated, it leads to end stage heart failure and ultimately death. Valve replacement has become a routine operation in the developed world for patients shown to have heart valve disease who have not yet reached the stage of irreversible, end stage heart failure.
In the past, there have been two broad types of valves that have been used in replacement procedures: mechanical valves and biological valves.
Mechanical valves are constructed from rigid materials. The design of these valves takes one of three general forms: ball and cage, tilting disk or bileaflet prostheses. In general, mechanical valves have in their favor long term durability intrinsic to the very tough materials from which they are made. With a few notable exceptions, such as the well publicized Shiley CC series, mechanical failure of these valves has been very rare. Followup for some of the first generation ball and cage valves now exceeds thirty years and the longevity of more recent designs such as the latest bileaflet prostheses is expected to match these results.
The principal shortcomings of mechanical valves, however, are the need for long term anticoagulation, the tendency to cause red blood cell haemolysis in some patients and the noise created by repeated opening and closing of the valve which patients find very disturbing. Anticoagulation requires the patient to take a regular daily dose of medication that prolongs the clotting time of blood. The exact dose of medication, however, needs to be tailored to the individual patient and monitored regularly through blood tests. Apart from the inconvenience and potential for non-compliance imposed by this regimen, inadvertent over-coagulation or under-coagulation is not uncommon. Under-coagulation can lead to thrombosis of the valve itself or embolism of clotted blood into the peripheral circulation where it can cause a stroke or local ischaemia, both potentially life threatening conditions. On the other hand, over-coagulation can cause fatal spontaneous haemorrhage. It is clear therefore that anticoagulation, even in the most expert hands, is associated with finite risks of morbidity and mortality. This risk accrues significantly over the patient""s lifetime. For this reason, some surgeons avoid the use of mechanical prostheses, where possible.
Hemolysis is the lysis of red blood cells in response to stresses imposed on those cells as blood crosses mechanical valves. Significant hemolysis causes anemia. These patients are required to have regular replacement blood transfusions with the attendant inconvenience, expense and risks which that entails.
Haemolysis and the need for anticoagulation result principally from microcavitation and regional zones of very high shear stress created in the flow of blood through mechanical valves. These physical phenomena are imposed on elements in the blood, i.e. red blood cells and platelets, responsible for activating the clotting cascade occasioned by the design of existing prostheses having either a rigid ball and cage, a rigid disk or two rigid leaflets.
Finally, mechanical valves may not be suitable for small patients as a significant gradient exists across these valves in the smaller sizes.
Biological valves are constructed from a variety of naturally occurring tissues taken from animals and fixed by treatment with glutaraldehyde or similar agent. Materials that have been used include dura mater from the lining of the brain, pericardium from the sac lining the heart or valve tissue itself from pigs and cows. These materials are used to fashion replacement heart valve leaflets and in the past have been assembled with the aid of a rigid supporting frame or stent. More recently leaflets made from these materials have been supported without the aid of a rigid frame and are fixed over flexible materials such as Dacron. The latter are referred to as stentless valves.
In contradistinction to mechanical valves, biological valves have flow hemodynamics that resemble the flow through native heart valves. In general, they do not therefore require lifelong anticoagulation and do not cause red cell hemolysis. Furthermore, very little residual gradient can be measured across even the smallest available stentless biological valves. Additionally, biological valves function inaudibly.
Unfortunately, however, biological valves suffer from degenerative changes over time. At least 50% of porcine valves implanted in the aortic position fail within 10-15 years post operatively. Furthermore, this risk is amplified in the mitral position and in younger patients where failure of porcine aortic valves is almost universal by five years. Progressive deterioration of biological valves manifests itself either as obstruction to forward flow through the valve in the open position, i.e. stenosis, or more commonly as tears in the valve leaflets that cause leakage in the closed position, i.e. regurgitation.
To summarize, the configuration of biological valves allows them to function inaudibly without the risks of thrombosis or hemolysis. However, the biological materials from which they are made do not have the durability to last the patient""s potential lifetime.
A valve that combines the durability of man-made materials with the hemodynamics of a biological valve would be inaudible, free from the problems of anticoagulation and risk of hemolysis and yet exhibit the necessary durability to last the patient""s lifetime.
This is the principal underlying development of stentless valves for the aortic or pulmonary position made from the elastomeric material, polyurethane. These valves do indeed exhibit favorable hemodynamics and have not thus far required anticoagulation. Accelerated fatigue testing has however shown that these valves do still suffer from degenerative changes in the longterm. As in the case of biological valves, degenerative changes in the materials that make up the leaflets are focused on local areas of high stress in the valve leaflets themselves and mechanical failure, not surprisingly, occurs at these exact same points. Mechanical failure is therefore a problem intrinsic to the leaflet material itself.
Polyurethane and other elastomeric polymers are isotropic when assembled as sheets i.e. they exhibit the same properties in all directions and at all points over the surface of the material. Reinforcement of elastomers with fibers improves their ability to withstand stress.
By way of further background, it will be noted that in the so-called Oxford valve, a regular uniform pattern of reinforcement is used in terms of Melinex sheets made from silicone reinforced with Terylene polyester. These were fabricated as flat sheets and then assembled as leaflets over a metal frame. The flat sheets tend to buckle in the closed position of the valve. This is because a normal valve leaflet has two axes of curvature. It is not possible to fashion a flat sheet into a surface with two axes of curvature without it buckling as is demonstrated in their valve.
In terms of the valve of Wheatley et al, European Journal of Cardio-thoracic Surgery 2000; 17:440-448, their valve is molded into curved sheets but is not reinforced. This valve showed significantly lower tendency to form blood clots than mechanical valves and improved durability over biologic valves. However, accelerated fatigue testing demonstrated calcification at wear-induced defects in the leaflet material, which were sites of subsequent material failure.
In accordance with U.S. Pat. No. 4,731,074 issued to Rousseau et al reinforced fabric is used with fibers oriented only in one direction to provide improved strength. However, this does not address the different direction and magnitude of stresses experienced in different regions of the leaflet.
The synthetic fiber reinforced stentless heart valve described by Cacciola et al, Journal of Biomechanics 2000;33:521-530, utilizes a mesh reinforcement in more than one direction. However, the patterns that they describe are regular patterns and are not specifically aligned with respect to stress lines. Such a regular matrix or mesh cannot address the regional variations in stress that exist over the entire valve leaflet due to their regularity.
In the present invention, the valve leaflets are made from composite materials and assembled in the geometric form of the native biologic valve. In the case of the aortic and pulmonary valves this is a stentless structure with valve leaflets supported respectively by the wall of the aorta or pulmonary artery only. In the case of the mitral or tricuspid valves the leaflets are supported by an annulus and additionally by chordae that extend from the free edge of the valve leaflet to the wall of the ventricle. In the case of a stented valve, for implantation into any of the foregoing anatomical positions, the leaflets are supported on a wire frame or stent to which is attached a sewing ring.
In the subject invention, mechanical properties of the leaflet material are optimized by tailoring reinforcement to those areas where it is needed, with the reinforcing strands configured in a direction and density that addresses differences in the magnitude and direction of stresses in different parts of the valve leaflet. The result of fabrication of the material over curved molds and with reinforcement of the material along lines of stress is the elongation of the lifetime of the valves by at least three times, making it unnecessary to replace the valves in the normal lifetime of an individual.
In one embodiment, the valve leaflet includes a laminated structure fabricated over a mold with two axes of curvature, with one or more layers of uninterrupted yarns, strands or fibers disposed in a continuous trajectory from one edge of the leaflet to the other edge.
The entire purpose of the reinforcing yarns, strands or fibers is to improve the fatigue resistance of the valve with fiber reinforcement in a density and direction to handle regional variations in the stresses experienced across the valve leaflet.
The principals described herein for the assembly of leaflets for replacement aortic or pulmonary valves can equally be applied to the assembly of valves for the mitral or tricuspid positions. In these latter valves the reinforcing fibers are disposed across the leaflet along lines of stress some of which continue as chordae that attach the free margin of the leaflet to the ventricular septum or free wall. Alternatively, the principals may be applied to the assembly of leaflets for use in a stented replacement heart valve.
In summary, a material for the construction of heart valve leaflets is provided through the use of oriented fiber components in a laminated composite wherein fibers are aligned along lines of stress in the material, thus to engineer fatigue resistance into the material and provide a long-lived valve that will function for the life of the patient. In a preferred embodiment, the reinforcing materials are optimized in terms of orientation of the fibers and in terms of their density. A valve constructed with flexible leaflets such as these will not require anticoagulants as is the case with mechanical valves or exhibit hemolysis in which red blood cells are damaged by the action of mechanical valves. Longevity exceeds thirty-five years in most cases, making replacement of such a valve a remote possibility. In one embodiment, oriented fiber components are provided by laying fibers in specific orientations over curved molds to which polymer sheets are laminated.